In a tube-type color camera the effective exposure time is 1/30th of a second, comprised of two interlaced fields of 1/60th of a second each. One of the drawbacks in tube-type cameras is that they exhibit low resolution when picking up fast moving objects (i.e., they have low dynamic resolution). In the past an optical shutter has been evaluated in such cameras to shorten the exposure time, however, the results have not been impressive due to the lag characteristics of tube-type sensors.
A solid-state imager typically has zero lag and, therefore, is a good sensor for picking up fast moving objects. With such a zero lag device it is now feasible to use optical shuttering to enhance the dynamic resolution of cameras using such devices. In a frame transfer (also known as a field-transfer) charge-coupled device (CCD) imager, it is generally considered to be necessary that the imager be optically shuttered during the interval (vertical blanking) in which the charge accumulated in the imaging area of the device is transferred to the storage area of the device. Failure to shutter during this period causes vertical image smear, most noticeable when picking up scenes containing highlights. In a frame transfer CCD imager, the effective exposure time is 1/60th of a second due to the method of signal readout. Interposing a mechanical shutter (e.g., a rotating wheel) between the main camera lens and the prism system can eliminate frame transfer smear and can also achieve an amount of optical shuttering during a charge accumulation interval of the CCD imager which enhances the dynamic resolution of the camera.
In U.S. patent application Ser. No. 575,570, filed on Jan. 31, 1984, in the names of S. L. Bendell and the present inventor, a solid-state imager camera is disclosed which includes a shutter arrangement for controlling the exposure time of the imagers. In accordance with this prior arrangement, a first shutter including opaque areas is rotated to block radiant energy from reaching the imagers during a fixed, relatively narrow, interval during the charge transfer period of the imager for eliminating vertical smear. A second shutter is arranged in series with the first shutter such that the combined effective width of the shutters may be adjusted to improve the resolution of moving images. With this arrangement, the light passage by the shutter system can be reduced to a slit, to enhance the dynamic resolution of the camera to a point where "true" stop motion of an image during action sequences can be realized. Furthermore, such a shutter system permits automatic exposure time control of a solid-state imager camera. The two shutters are arranged coaxially. One shutter motor is locked to vertical sync while the other shutter motor, which is also locked to vertical sync, is provided with a variable phasing control. Variation of the relative phase of the two shutters changes the effective exposure time.
A variable shutter exposure control permits the camera operator to select different exposure techniques for artistic or special-effect purposes. Unfortunately, as the exposure time is reduced, the charge accumulation of the CCD photosensor is also reduced, lowering the light sensitivity performance of the camera (i.e., the low light level performance is degraded). In a television camera including such a shutter system, it is desirable to be able to selectively improve the light sensitivity of the camera, especially under low light level conditions.